1. Field of the Invention
The present invention relates to a system and a method for controlling the actuation of a compressor and particularly a compressor applied to cooling systems in general, this system and method enabling one to eliminate the use of thermostats or other means of measuring temperature usually employed in this type of system.
2. Description of the Related Art
The basic objective of a cooling system is to maintain low temperature inside one (or more) compartment(s), making use of devices that transport heat from the interior of this (these) environment(s) to the external environment. It uses the measurement of the temperature inside this (these) environments to control the devices responsible for transporting heat, trying to keep the temperature within limits pre-established for the type of cooling system in question.
Depending upon the complexity of the cooling system and of the kind of application, the temperature limits to be maintained are more restricted or not.
One usual way of transporting heat from the interior of a cooling system to the external environment is to use a hermetic compressor connected to a closed circuit through which a cooling fluid circulates, wherein the compressor has the function of providing the flow of cooling gas inside the cooling system, being capable of imposing a determined difference in pressure between the points where evaporation and condensation of the cooling gas occur, whereby it enables the processes of transporting heat and creating low temperature to take place.
The compressors are sized to supply a capacity of cooling higher than that required in a normal situation of operation, foreseen critical situations of demand. In this case, some type of modulation of the cooling capacity of this compressor is necessary to maintain the temperature inside the cabinet within acceptable limits.
The most usual way of modulating the cooling capacity of a compressor is to turn it on and off according to the evolution of the temperature in the environment being cooled, by making use of a thermostat that turns the compressor on when the temperature in the environment being cooled exceeds a pre-established limit, and turns it off when the temperature in this environment has reached a lower limit, also pre-established.
The known solution for this device of controlling the cooling system is the use of a bulb containing a fluid that expands and contracts with temperature, installed in such a way that it will be exposed to the temperature inside the environment to be cooled and mechanically connecting an electromechanical switch that is sensitive to this expansion and contraction of the fluid inside the bulb. It is capable of turning the switch on and off at predefined temperatures, according to the application. This switch interrupts the current supplied to the compressor, controlling its operation, maintaining the internal environment of the cooling system within pre-established temperature limits.
This is still the most widely used type of thermostat, since it is relatively simple, but it has drawbacks such as fragility during the mounting, because this is an electromechanical device containing a bulb with pressurized fluid and also has limitation of quality due to the constructive variability and wear. This generates a relatively high cost of repair in the field, because it is linked to an equipment of high aggregate value.
Another known solution for controlling a cooling system is the use of an electronic circuit capable of reading the temperature value inside the environment being cooled, by means of a PTC-type (Positive Temperature Coefficient) electronic-temperature sensor, for example, or some other type. The circuit compares this read temperature value with predefined references, generating a command signal to the circuit that manages the energy delivered to the compressor, providing correct modulation of the cooling capacity, so as to maintain the desired temperature in the internal environment being cooled, be it by turning on or off the compressor, or by varying the delivered cooling capacity.
This solution provides a quite reliable and precise control of the temperature, further enabling one to perform more complex or additional functions. It is found in more sophisticated systems, which have a higher aggregate value.
A drawback is the relatively higher cost when compared with that of the electromechanical solution and, at best, with an equivalent cost for simple versions, when the device is employed in the basic function of keeping the temperature within certain limits.
Another solution for controlling the temperature in an environment being cooled is described in document U.S. Pat. No. 4,850,198, which discloses a cooling system that comprises a compressor, condenser, expansion valve and evaporators, besides providing control over energizing the compressor. This control is effected by means of a microprocessor in accordance with a temperature readout from a thermostat determining the energizing or no energizing of the compressor on the basis of the maximum and minimum predetermined temperature limits. According to this system, one still foresees control over time of operation of the compressor as a function of the temperate measured in the environment being cooled.